As has happened before (see tracker.php?p=2&t=18101
among others), Roasted Magikarp will not evolve into Zombie Gyarados like it should.
Was on a multiplayer server when I first noticed. I obtained a roasted magikarp by getting it struck by lightning, then gave it a rare candy to evolve it. A regular gyarados was the result.
Went into a singleplayer world to test. Gave myself a roasted magikarp through the pokemon editor, then gave it a rare candy to evolve it. Same result, a regular gyarados. I then checked the gyarados with the pokemon editor, and was able to give it the zombie texture that way. So, unlike previous times this has happened, the data for the texture is all there, except for whatever makes the roasted magikarp evolve into zombie gyarados.